1. Field of the Invention
This invention relates to a process of electrochemically decomposing sodium azide, and, more particularly, to a method of electrochemically decomposing sodium azide in aqueous alkaline solutions to form sodium hydroxide, ammonia, nitrogen, and oxygen. The invention further relates to an apparatus for the electrolytic reduction of sodium azide.
2. Description of the Related Art
Sodium azide has been widely used in the automotive as well as other manufacturing industries. The facilities that use sodium azide often generate aqueous alkaline side streams containing this chemical which must be removed or chemically altered before waste water streams can be discharged from the facilities.
The sodium azide manufacturing process also generates an aqueous alkaline side stream containing a high concentration of sodium hydroxide (up to 60% of sodium hydroxide on a weight basis) and a low concentration of sodium azide (approximately 2% on a weight basis). The sodium hydroxide generated from this side stream can be sold as a product only if the sodium azide in this side stream has first been removed or chemically changed to a non-hazardous material.
Azide compounds in aqueous solutions are typically decomposed by treatment with chemicals, electrolysis, ultraviolet light, or ozonation. Most of the methods dealing with azide decomposition have severe limitations because they are hazardous, expensive, inefficient, of limited applicability, and because they generate hazardous waste. Some of the methods further require operation at extremely high temperature and/or in solutions with low to moderate pH, which methods are not suitable for the decomposition of sodium azide in aqueous alkaline side streams.
Chemical decomposition of azide often generates unwanted hazardous waste. For instance, Heubner et al. (U.S. Pat. No. 5,457,265) describe a method that involves oxidative degradation of azide by treating the azide-containing solutions with an iodine solution in the presence of iodide and a thiosulfate. Although Heubner's invention is said to work well at an extremely low concentration of azides, it is not suitable for treating aqueous alkaline side streams because the chemicals used in the process, namely, iodine and thiosulfate, would further contaminate the waste stream.
Electrolytic reduction of azide compounds is another commonly used method for the treatment of azide compounds in aqueous solutions. For example, Andrus (U.S. Pat. No. 4,004,994) describes an electrochemical apparatus that utilizes carbonaceous particles (including carbon itself, charcoal, graphite, and pelleted forms of such carbonaceous materials) to form a solid particulate bed. Azide, a non-metallic ionic contaminant, can be removed from the aqueous liquid by passing the liquid to be treated into the base of a vertically disposed treatment zone containing the solid particulate bed which forms a medium of low electrical conductivity. The treatment zone is formed by a cylindrical electrode and at least one internal electrode axially disposed within the cylindrical electrode. The liquid, free of contaminants, is then removed from the top of the treatment zone. Andrus' process, however, is not suitable for the treatment of contaminants in highly alkaline solutions because the carbonaceous particles would be subjected to significant degradation and oxidation which would contaminate the solution being treated.
Polson (U.S. Pat. No. 4,236,982) describes an electrolytic process of removing lead azide in an aqueous alkaline electrolyte containing 10% to 20% (weight basis) of sodium hydroxide, 0.2%-0.6% of rosin powder, and 5% of sodium-potassium tartrate. Although azide can be decomposed and converted to nitrogen gas at the anode of the electrolytic cell, the main purpose of the Polson process is to destroy lead azide by plating lead at the cathode. Also, the Polson process requires operation at a fairly high temperature (about 180.degree. F.) which may create potential hazards. In fact, the Polson process is specially designed to remove a large quantity of lead azide in the electrolyte. There is no indication that Polson's process can be effective to reduce the concentration of azide compounds in the solution to a non-detectable level as in the present invention in part because the operation conditions may not be useful in a lead-free solution. For example, the current density preferred in Polson is 0.046 to 0.085 amp per square inch. Although higher current densities can be used to increase yield, according to Polson the quality of lead produced at these high densities is diminished. The deposition of lead would be detrimental to cathodes used according to the invention and especially a cathode in contact with .TM.Raney Nickel catalysts.
There are also several other reports relating to the electrochemical treatment of specific types of electrolyte where sodium azide is incidentally decomposed in the process. For example, Bones et al. (U.S. Pat. No. 4,774,156) provide a rechargeable electrochemical cell where sodium azide is employed to generate nitrogen gas which would pressurize the electrode compartments to force liquid into the electrode chambers. Bones et al.'s invention requires extremely high temperature (320.degree. C.) to decompose the sodium azide. Sodium azide is not the targeted chemical to be removed from the electrolyte in Bones' system. Lippman et al. (U.S. Pat. No. 5,423,454) describe another electrochemical process for the generation of a gas to form an internal pressure to dispense a product. A water soluble azide (e.g., sodium azide) is used to generate nitrogen gas in Lippman's process. The inventions described by Bones and Lippman, although both involving the decomposition of sodium azide, are not intended to be used for the removal of sodium azide from the solution since sodium azide is one of the reactants which have to be added to the systems.
Ozonation is also a popular way of removing azide compounds from gaseous and aqueous waste. For example, Clausen et al. (U.S. Pat. No. 5,433,932) and Gupta et al. (U.S. Pat. No. 5,073,273) both describe methods of treating gas generating material waste that contains alkali metal azides (e.g., sodium azide) with ozone gas to oxidize the alkali metal azide to nitrogen and alkali metal nitrate. However, ozonation has to be carried out at a low pH (10.0-12.5). Therefore, it would not be feasible for the treatment of sodium azide in highly alkaline solutions.
It is therefore an object of the invention to electro-chemically decompose sodium azide in an efficient way and to recover or remove chemicals from waste water streams.